Visual Studio: Edit XAML file while debugging
Right-click the XAML file in Solution Explorer and select Open With..., then choose Source Code (Text) Editor and click Set as Default. You can then edit XAML files while the app is running, without detaching. You'll need to reopen any XAML files before this works. Tested in VS2012.
FYI, I'm able to make changes in VS2017 XAML, and most changes even update the UI live (merged RDs don't seem to work). I can't remember if I had to disable Edit and Continue (been a default step of mine going back to its first appearance, since I've never had luck getting it working like I expect it to). I'm going back in 2015 trying it, and it's not letting me do it (IIRC 2015 and maybe 2013 were the only ones to totally lock the XAML)